(1) Field of the Invention
This invention relates to a method which comprises moving a plate-like or flat preform obtained by injection molding a synthetic resin into a mold to a blow mold, as it is, and applying stretching by a rod and expansion by air pressure to the preform into a container which is wide in opening and has a thin-wall.
(2) Description of the Prior Art
It is already known that a flat preform subjected to injection or injection compression molding is molded into a cup-like or box-like thin-wall container by making use of blow molding or vacuum molding.
This molding method requires to effect blowing or vacuum molding while holding a peripheral edge of a flat preform, and said holding force need be extended over the whole peripheral edge. If there is a part which partly lacks in holding force, said part is slipped out of a holding mold by the blowing force or attracting force to fail to mold a well-shaped thin-wall container.
In view of the foregoing, in the conventional method, upwardly directed projections are provided within a lateral groove formed in the peripheral wall of a cavity of a mold for molding a preform, and the projections are forced into the peripheral edge of the preform when the latter is molded to serve as a stop during molding a container.
With such a stop, a molded container cannot be removed from a preform molding mold as it is, and therefore, the preform molding mold is made to comprise two upper and lower members placed one above the other so as to be moved to and from each other, and said lateral groove is formed between the upper member and the lower member. In removing a container, the lower member is moved downwardly to open the lateral groove so that the peripheral edge of the preform is moved away from the projection and then removed.
Therefore, the lateral groove need be operated to open and close vertically every time a container is molded, and opening is carried out with the lower member moved downwardly where the container molding mold is positioned. Therefore, the positional relation with respect to the container molding mold also need be taken into consideration, which poses a drawback in that construction of the preform molding mold also becomes complicated.
In addition, since the projection is forced into the peripheral edge when the preform is molded, there poses a further disadvantage that the height of the lateral groove has a limitation, the peripheral edge increases its thickness more than as needed, a mark formed by the projection forced-in remains as an annular groove, and as a result the peripheral edge tends to be bended.